This invention relates to millimeter wave filters, and more particularly, this invention relates to millimeter wave filters such as parallel coupled line filters.
High performance millimeter wave (MMW) filters have typically been designed and fabricated using thin film technology by techniques known to those skilled in the art. Any manufacturing techniques using thin film technology requires tight design tolerances to achieve a desired filter response at various millimeter wave frequencies. These tolerances include the necessary and critical dimensions concerning material thickness, surface roughness, dielectric constants, metallization thicknesses, and transmission line width and spacing.
In these prior art thin film filter designs, the filter length and filter width usually varied as a function of the frequency band because of the wavelength change. Any change in the filter length made it difficult to design a common radio frequency module layout across multiple frequency bands. Because these high frequency thin film technology filters have been expensive, they were usually fabricated separately and then attached as a bare semiconductor die to a carrier plate or directly to a housing using epoxy or solder.
As the millimeter wave industry has moved closer to implementing high volume surface mount manufacturing techniques, there has been a need for low cost surface mount filters that can be used at high frequency. It would be advantageous if low cost, high performance millimeter wave filters could be manufactured using one or more layers of thick film, low temperature co-fired ceramic technology (LTCC) with the associated lower manufacturing tolerances. Any millimeter wave filter using this technology would have to achieve high xe2x80x9cQxe2x80x9d filters in small spaces.
Microstrip and stripline interface methods are possible approaches. Any filters manufactured using these low temperature co-fired ceramic materials should be desensitized to the traditional critical tolerances associated with thin film technology and compensate for any bandwidth and return loss degradation caused by wider tolerances that are associated with thick film technology. The advantages would be a filter that is produced at a fraction of the cost of thin film filters.
The present invention advantageously provides a high performance millimeter wave filter using low temperature co-fired ceramic thick film technology. It achieves high xe2x80x9cQxe2x80x9d filters in a small space by vertically stacking resonators in a multilayer low temperate co-fired ceramic film. These resonators can form parallel coupled line filters, including a hairpin filter. Microstrip and stripline interface connections are used to stack the filters in the low temperature co-fired ceramic layers, allowing the structure to be used for standard surface mount packages. The filters are desensitized to traditional critical tolerances associated with thin film technology and compensate for bandwidth and return loss degradation caused by wider tolerances associated with the thick film technology. These type of filters can be manufactured for high performance capabilities at a fraction of the cost of thin film filters. Additionally, these filters, including hairpin filters, can eliminate filter size variation versus frequency and reduce the size of the filter by fifty percent.
In accordance with one aspect of the present invention, the millimeter wave filter for surface mount applications includes a dielectric base plate having opposing surfaces. A ground plane layer is formed on a surface of the dielectric base plate. At least one low temperature co-fired ceramic layer is positioned over the ground plane layer and defines an outer filter surface. A plurality of coupled line millimeter wavelength resonators, such as parallel coupled resonators, including hairpin resonators, are formed as stripline or microstrip and positioned on the outer filter surface.
Radio frequency terminal contacts are positioned on the surface of the dielectric base plate opposite the at least one low temperature co-fired ceramic layer. Conductive vias extend through the at least one low temperature co-fired ceramic layer, ground plane and dielectric base plate and each interconnect radio frequency terminal contacts and at least one coupled line resonator.
In yet another aspect of the present invention, a lower ground plane layer is positioned on the surface of the dielectric base plate opposite the ground plane and the ceramic layer and isolated from the radio frequency terminal contacts. A plurality of isolation vias extend through the low temperature co-fired ceramic layer and dielectric base plate and engage the lower ground plane layer.
In still another aspect of the present invention, a plurality of low temperature co-fired ceramic layers and interposed ground plane layers form a multilayer low temperature co-fired ceramic substrate board. A plurality of millimeter wavelength stripline resonators are formed on the ceramic layers between the outer filter surface and dielectric base plate and isolated by the interposed ground plane layers. In one non-limiting example, these resonators are hairpin resonators. Conductive vias interconnect resonators formed on the ceramic layers and outer filter surface. The resonators can form two-pole hairpin filters and can be about one-quarter wavelength long. A dielectric cover is positioned over the outer filter surface and has a metallized interior surface spaced from the hairpin resonators for generating a predetermined cut-off frequency. This dielectric cover is spaced about 15 to about 25 mils distance from the resonators formed on the upper filter surface. Resonators formed on the outer filter surface can be formed as microstrip.
In another aspect of the present invention, the dielectric base plate is formed from a ceramic material, such as an aluminum oxide, which is about 10 to about 35 mils thick.